1. Field of the Invention
The present invention relates to an optical pickup apparatus performing operations for reading signals recorded in optical discs
2. Description of the Related Art
Optical disc apparatuses are widely used each of which may apply a laser beam emitted from an optical pickup apparatus to a signal recording layer provided in an optical disc to perform a signal reading operation.
An operation of reading signals recorded on the signal recording layer by an optical disc apparatus is performed by applying a laser beam emitted from a laser diode to the signal recording layer and detecting changes in the laser beam reflected from the signal recording layer with a photodetector.
In reading signals recorded on the signal recording layer with a laser beam, accurate executions is needed of a focusing control operation of focusing the laser beam onto the signal recording layer and the tracking control operation of causing the laser beam to follow a whorl-like signal track provided on the signal recording layer.
Although various methods exist for executing the focusing control operation, an astigmatic method utilizing generation of astigmatism is generally known. Although various tracking control methods also exist, a push-pull method and a phase difference method are generally known. Recently, a laser beam emitted from the laser diode is split into three beams, i.e., a main beam and two sub-beams, through diffraction by a diffraction grating, and focusing control methods and tracking control methods using not only the main beam but also the sub-beams are the mainstream.
Recent optical pickup apparatuses use such a main beam and sub-beams to employ a differential astigmatic method as the focusing control method and to employ a differential push-pull method and differential phase difference method as the tracking control method. When such a main beam and sub-beams are used, the optical pickup apparatus is provided with a photodetector including two sub-beam light-receiving portions to which two sub-beams are respectively applied and a main beam light-receiving portion to which a main beam is applied, and is configured to perform control operations by generating a focus error signal and a tracking error signal from signals obtained from the photodetector. Such a technology is well-known and will not be described.
Recently, optical discs provided with two layers instead of one layer are commercialized, and optical pickup apparatuses are also commercialized that are capable of performing operations of reading signals recorded on the signal recording layers of such optical discs.
When performing the operation of reading signals recorded on a two-layer optical disc, the focusing control operation and the tracking control operation are performed for one signal recording layer being subjected to the reading operation, however, the laser beam is also reflected from the other signal recording layer that is not subjected to the reading operation. The laser beam reflected from the signal recording layer not being subjected to the reading operation is generally referred to as stray light, and it is problematic that such stray light is applied to a photodetector generating a focus error signal to be used for the focusing control operation using the differential astigmatic method, and is caused to generate offsets or amplitude variations in the focus error signal, and thus the focusing control operation cannot accurately be performed.
A method for solving such a problem of stray light has been developed, whereby a shape of the photodetector is changed into a shape for eliminating an effect of stray light (see Japanese Patent Application Laid-Open Publication No. 2007-42236).
Although an art disclosed in the above Japanese Patent Application Laid-Open Publication No. 2007-42236 restrains the effect of stray light on a multi-layer optical disc provided with a plurality of signal recording layers, an area of a light-receiving portion for receiving the main beam is set to be substantially the same as an area of a light-receiving portion for receiving the sub-beams, and thus, the art is easily affected by the stray light generated from the main beam, which is a problem.
The focusing control operation in the optical disc apparatus is performed through a combination of an operation of greatly displacing the objective lens included in the optical pickup apparatus to an operational position, i.e., a position for focusing the laser beam onto the signal recording layer and a following operation of minutely displacing a position of an objective lens to correct focusing deviation caused by plane vibrations of the optical disc.
When the objective lens is displaced to the operational position in such a focusing control operation, since a laser spot is greatly expanded that is applied to the sub-beam light-receiving portions and the main beam light-receiving portion included in the photodetector, a spot of an unnecessary laser beam, especially, the main beam with higher light intensity is applied to the sub-beam light-receiving portions, and thus, the focusing control operation is adversely affected, which is a problem.